1. Field of the Invention
The present invention relates to a high frequency oscillator for use in a microwave band or a millimeter band, and more particularly, to a circuit configuration which can implement a push-push oscillator for generating a signal at a frequency twice as high as an oscillation frequency and a push-pull oscillator for combining oscillation outputs, and improve the performance of these oscillators.
2. Description of the Related Arts
A high frequency oscillator for use in a microwave band or a millimeter band is a core component among others in high frequency hardware, and simultaneously accounts for a large proportion of the device cost in the high frequency hardware, so that higher performance and economization are required for the high frequency oscillator. Particularly, a high frequency oscillator for use in a millimeter-wave region of 30 GHz or higher experiences lower oscillation power and deteriorated noise characteristic associated with a degraded performance of semiconductor devices and an increased circuit loss, and encounters difficulties in a reduction in the cost because a highly accurate assembling process, a high frequency testing process, and the like are required for manufacturing such a high frequency oscillator. These disadvantages also constitute main factors which impede the progress in applications of radio waves in the millimeter band. Remedies contemplated for the impediment may include the use of multiple oscillator elements, power combination based on a push-pull operation, oscillation of a high frequency twice as high as a fundamental frequency through push-push oscillation, and the like.
FIG. 1 illustrates the configuration of a conventional high frequency oscillator. This high frequency oscillator comprises dielectric resonator 2X as a high frequency (microwave) resonator installed on substrate 1; two active devices 3 for oscillation each including negative resistance elements, amplifier elements, and circuits associated therewith; and combiner circuit 4. Two active devices 3 each employ, for example, an FET (field effect transistor) as an amplifier element, and share dielectric resonator 2X to form two oscillation systems. A ground conductor (not shown) is deposited substantially over the entire back surface of substrate 1.
Each FET 3 has a gate connected to one end of associated microstrip line 5. These microstrip lines 5 are positioned on both sides of dielectric resonator 2X and electromagnetically coupled to the same. Each microstrip line 5 has the other end grounded through terminal resistor 10. FET 3, the source of which is grounded, generates an oscillation output at its drain Here, the respective oscillation systems are designed to oscillate in opposite phase to each other, so that two FETs 3 provide oscillation outputs in opposite phase at their drains.
The drains of respective FETs 3 are connected to combiner circuit 4. Combiner circuit 4 comprises a Wilkinson type power combiner circuit when it is designed, for example, for in-phase combination, and additionally includes a phase inversion circuit connected to one input of the Wilkinson type power combiner circuit when it is designed for antiphase combination. Combiner circuit 4 receives and combines respective oscillation outputs from the drains of the active devices for oscillation, i.e., FETs 3.
For example, for configuring a push-push oscillator which generates oscillation frequency 2f0 twice as high as oscillation frequency f0 in each oscillation system, combiner circuit 4 receives oscillation outputs from respective FETs 3 as they are, and combines the oscillation outputs in phase. In this event, since FETs 3 themselves have oscillation outputs in opposite phase to each other, the fundamental component f0 and its odd-numbered harmonic components in the oscillation output of each oscillation system are canceled out, while second-order and higher even-numbered harmonic components are combined and delivered from the output of combiner circuit 4. FIGS. 2A to 2C show the waveforms observed around combiner circuit 4 when the circuit illustrated in FIG. 1 is used as a push-push oscillator. FIGS. 2A and 2B show the waveforms at two input terminals of combiner circuit 4, respectively, while FIG. 2C shows the waveform at the output of combiner circuit 4.
When the high frequency oscillator illustrated in FIG. 1 is designed to function as a push-pull oscillator for generating combined output Po from the oscillation outputs of both oscillation systems at an output frequency remaining equal to fundamental frequency f0, combiner circuit 4 is configured to perform an antiphase combination. Specifically, for the antiphase combination, combiner circuit 4 combines the oscillation output from one FET 3, after it is inverted, with the oscillation output from the other FET 3 which remains as it is. In this way, since the oscillation outputs are combined with fundamental wave f0 remaining unchanged, combiner circuit 4 generates combined output Po which has the oscillation frequency equal to fundamental wave f0 and an increased amplitude.
As described above, this type of high frequency oscillator which operates as a push-push oscillator or a push-pull oscillator comprises high frequency (microwave) resonator 2; two active devices 3 for oscillation (e.g., FET); and combiner circuit 4 for performing an in-phase or an antiphase combination. FIG. 3 illustrates this configuration in block diagram form. As is apparent from this block diagram, in the conventional high frequency oscillator, high frequency resonator 2 and combiner circuit 4 are designed as completely different components. This designing policy results in a correspondingly complicated circuit configuration. With the trend of increasingly higher oscillation frequency, a more complicated design is generally required for the three components, giving rise to difficulties in reducing the size of the circuit. Combiner circuit 4 disposed in the high frequency oscillator also causes a larger increase in loss.
The aforementioned Wilkinson type combiner circuit routes lines for combining two oscillation outputs to form the combiner circuit, giving rise to a problem that a complicated design is required therefor including an oscillator circuit.
Moreover, if mutually synchronized oscillations can be realized, for example, at four phase angles, i.e., synchronized oscillations with mutual phase differences being at 0 degree, 90 degrees, 180 degrees and 270 degrees, it is possible to generate an oscillation output at a frequency four times as high as the oscillation frequency (fundamental wave f0) in oscillation systems based on the principles of the push-push oscillator. However, any four-phase push-push oscillator has not been reported up to now. In other words, it is contemplated that the conventional circuit configuration can hardly be based to design this type of four-phase push-push oscillator.
It is an object of the present invention to provide a high frequency oscillator which is suitable for reducing the size and loss of the circuit and is capable of realizing, for example, even a four-phase push-push oscillator.
The object of the present invention is achieved by a high frequency oscillator which includes a transmission line resonator; a pair of active devices for oscillation connected to a pair of resonant wave points, respectively, located on the transmission line resonator in an opposite phase relationship to each other, and a combiner line connected to an electric symmetric point of the transmission line resonator. The pair of active devices constitute a pair of oscillation systems which share the transmission line resonator as a high frequency resonator, and the combiner line is configured to combine outputs from the pair of oscillation systems to generate a high frequency output.
In the present invention, the active devices for oscillation are connected, for example, to two resonant wave points located on the transmission line resonator in an opposite phase relationship to each other, so that the high frequency oscillator includes two oscillation systems which oscillate at the same oscillation frequency in opposite phase to each other. The resonant wave point is, for example, an antinode of the oscillation. In addition, since the combiner line is connected to the electric symmetric point of the transmission line resonator, the outputs of the respective oscillation systems can be combined in phase or in opposite phase. This combiner line also functions as an output line. For example, when the high frequency oscillator is designed such that the combiner line combines the outputs of the two oscillation systems in phase, the combiner line delivers double wave component 2f0 while canceling out fundamental wave components f0 of oscillation in the respective oscillation systems. Consequently, a push-push oscillator is provided. On the other hand, when the high frequency oscillator is designed such that the combiner line combines the outputs in opposite phase, the combiner line delivers combined output Po of fundamental waves f0. Thus, a push-pull oscillator is provided. In the following description, a second-order harmonic of fundamental wave f0 is called the xe2x80x9cdouble wavexe2x80x9d, and a fourth-order harmonic is called the xe2x80x9cquadruple wavesxe2x80x9d.
Describing in greater detail, when the active devices are connected at positions at which the transmission line resonator resonates at phases which are opposite to each other, the high frequency oscillator can induce antiphase oscillations which mutually involve phase synchronization, i.e., mutually synchronous oscillations. Any resonator having physical symmetricity always includes an electric symmetric point or an electric symmetric plane in its dipole proper wave field which is used for oscillation, so that an in-phase combination can be readily implemented only by connecting a combiner line to the electric symmetric point or plane.
This means that the high frequency oscillator need not be provided with a separate in-phase combiner circuit required for a push-push operation but can extremely simply implement an in-phase combination function. What should be taken into account in the design is only a setting of a degree of coupling between the resonator and oscillation output terminal. This is equivalent to integration of a resonator with a combiner circuit. The integration is not only significantly effective in simplification and reduction of the circuit configuration but also contributes to easy implementation of even a four-phase push-push oscillator, as later described.
Near the electric symmetric point or electric symmetric plane of a resonator, an antiphase combiner circuit can also be accomplished with the ensured electric symmetricity. Likewise, in this configuration, a so-called push-pull oscillator operation can be readily implemented even in a high frequency region such as a millimeter band without separately providing an antiphase combiner circuit.
Conventionally, a high frequency resonator is separate from a combiner circuit in configuration, whereas the high frequency resonator can be integrated with the combiner circuit only by forming or adding the combiner line to the transmission line resonator in accordance with the present invention. FIG. 4 illustrates in block diagram form the theoretical configuration of such a high frequency oscillator according to the present invention.
In the present invention, the high frequency resonator employed as the transmission line resonator is typically a microstrip line resonator. The microstrip line resonator is disposed on one principal surface of a substrate made of a dielectric material or the like, by routing a linear circuit conductor or a circular circuit conductor as a signal line in microstrip line structure. A ground conductor is deposited over the other principal surface of the substrate. Here, a resonator which has a linear circuit conductor employed for a signal line in microstrip line structure is called the xe2x80x9cline resonatorxe2x80x9d, while a resonator which has a circular circuit conductor for the signal line is called the xe2x80x9cring resonatorxe2x80x9d.
The combiner line is typically routed to function as a transmission line on the same substrate on which the transmission line resonator is disposed. The combiner line may be a microstrip line which is connected to the microstrip line resonator through a capacitor or directly without a capacitor. Alternatively, the combiner line may be a slot line routed on the other principal surface of the substrate and electromagnetically coupled to the microstrip line resonator through the substrate.
The high frequency oscillator according to the present invention as described above can extremely readily implement a push-push oscillator which is capable of generating an oscillation output at a frequency twice as high as oscillation frequency f0 in the respective oscillation systems. As appreciated, the present invention is effective in provide a higher performance but less expensive oscillator particularly for use in an ultra-high frequency band such as a millimeter band. The present invention also provides a push-pull oscillator in a quite simple configuration which is expected to generate higher power, so that the present invention is also promising particularly in the configuration of a multi-element power combination oscillator. Furthermore, the feature of the present invention which lies in the integration of the transmission line resonator with the combiner circuit also enables the origination of a four-phase push-push oscillator which has never been brought into realization.
Particularly, the enablement of the four-phase push-push oscillator means direct generation of an oscillation signal which has a frequency four times as high as a limit frequency imposed to a semiconductor device for use as an active device for oscillation. For example, if the oscillation frequency is limited to 300 GHZ due to the frequency characteristic of a semiconductor device, the four-phase push-push oscillator can generate a frequency signal at 1.2 THz which is four times as high as the limit frequency. In comparison with a frequency multiplication technique conventionally used for generating millimeter-waves, the high frequency oscillator according to the present invention can largely reduce the processing for removing unwanted waves, so that it is promising particularly as an ultra-high speed signal source appropriate to a high frequency band.